1. Field of the Invention
The present invention relates to an electrostatic latent image bearing member for use in electrophotography. In addition, the present invention relates to an image forming apparatus, a process cartridge, and an image forming method using the electrostatic latent image bearing member.
2. Discussion of the Background
In image forming apparatuses using electrophotography (such as copiers, printers, facsimiles), an image is typically formed as follows:                (1) a uniformly charged photoreceptor (i.e., electrostatic latent image bearing member) is irradiated by a light containing image information to form an electrostatic latent image thereon;        (2) a developing means supplies a toner to the electrostatic latent image to form a toner image on the photoreceptor;        (3) the toner image formed on the photoreceptor is transferred onto a recording medium (e.g., recording paper);        (4) a fixing means fixes the toner image onto the recording medium upon application of heat and pressure thereto; and        (5) residual toner particles remaining on the surface of the photoreceptor are removed with a cleaning blade and collected.        
In such electrophotographic image forming apparatuses, organic photoreceptors including an organic photoconductive material are widely used. Organic photoreceptors have the following advantages:                (1) capable of using materials responsive to various light (e.g., visible light, infrared light) irradiators, which are easily developed;        (2) capable of using environment-friendly materials; and        (3) low manufacturing cost.        
On the other hand, organic photoreceptors have poor mechanical strength, and therefore photosensitive layers thereof are abraded after long repeated use. When a specific amount of the photosensitive layer is abraded, the electrical property of the photoreceptor changes, and therefore a proper image forming process cannot be performed. The photoreceptor is abraded due to the friction between the photoreceptor and all image forming members (such as developing means, transfer means) which are in contact with the photoreceptor in an image forming process.
Various attempts have been made to prevent the photoreceptor from being abraded so as to lengthen the life thereof. For example, Japanese Patent No. (hereinafter referred to as JP) 3258397 discloses a photoreceptor having a protective layer including a hardened silicone resin containing a colloidal silica. It is described therein that such a protective layer has good abrasion resistance. However, fogging and blurring tend to appear in produced images after long repeated use because such a photoreceptor has insufficient electrophotographic property. Such a photoreceptor cannot satisfy the recent demands for a long-life photoreceptor having good durability.
JP 3640444 discloses a resin manufacturing method in which an organosilicon polymer is hardened in the presence of an organosilicon-modified positive hole transport compound. JP 3267519 discloses a photoreceptor having an outermost layer including a resin prepared by the above method. Such a photoreceptor tends to produce blurred images, and therefore an image-blurring-preventing mechanism such as a drum heater needs to be mounted on the machine used, resulting in upsizing of the machine and increasing the manufacturing cost. In addition, residual potential of the irradiated portion of the photoreceptor is hardly reduced, and therefore image density tends to decrease when the photoreceptor is particularly used for low potential developing processes.
Published unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 2000-171990 discloses a photoreceptor having a resin layer including a hardened siloxane resin having a charge transport group, which has a three-dimensional network structure. In such a photoreceptor, cracks tend to appear on the layer due to volume contraction of the resin, especially when low-priced and easy-to-handle commercially available coating agents are used in combination. In addition, residual potential of the irradiated portion of the photoreceptor depends on the layer thickness. Moreover, image density tends to decrease when the photoreceptor is used for low potential developing processes. When the content of the charge transport group increases, the layer strength decreases, and therefore durability of the photoreceptor deteriorates. Such a photoreceptor tends to produce blurred images after long repeated use. It is difficult to easily obtain a photoreceptor in low cost which can produce high quality images for a long period of time.
JP-A 2003-186223 discloses a photoreceptor having a protective layer including a charge transport material having at least one hydroxyl group, a three-dimensional cross-linked resin, and a particulate conductive material. It is described therein that such a photoreceptor has good abrasion resistance, and residual potential can be decreased to some extent. However, the particulate conductive material decreases volume resistance of the protective layer, and therefore blurred images tend to be produced due to blurred electrostatic latent images, especially under high temperature and high humidity conditions. Since the charge transport material may be a constitutional unit of the three-dimensional structure, as the amount of the charge transport material included in the protective layer increases, the effect of the molecular structure thereof (i.e., the number and the binding site of hydroxyl group) on abrasion resistance of the protective layer increases. In some cases, the resultant photoreceptor has insufficient abrasion resistance.
JP-A 2004-117766 discloses a photoreceptor having a protective layer including a urethane resin which is obtained by cross-linking plural polyols and a polyisocyanate. It is described therein that such a photoreceptor has good abrasion resistance. When an underlying layer (i.e., a recording layer) of the protective layer includes a polycarbonate, the adhesion between the protective layer and the underlying layer is not always sufficient. In this case, the protective layer tends to peel off from the edge of the photoreceptor or the portion on which scratches were made by carriers and paper powders, and therefore the underlying layer is exposed. Since a portion at which the underlying layer is exposed has charging property and light attenuation property different from those of an unexposed portion, abnormal images such as color unevenness tend to be produced.
When the thickness of the protective layer decreases due to abrasion, the protective layer easily peels off and disappears, resulting in reducing the life of the photoreceptor. In order to improve durability of the photoreceptor, the protective layer needs to have a large thickness. In this case, residual potential of the irradiated portion of the photoreceptor increases. When the residual potential is too high, potential gradation of the irradiated portion of the photoreceptor tends to deteriorate, and image density tends to decrease.
By the way, spherical polymerization toners come into practical use so as to respond to recent demands for producing high quality images. It is generally known that spherical polymerization toners remaining on a photoreceptor are difficult to remove with a cleaning blade made of a urethane rubber, compared to conventional pulverization toners. In attempting to solve this problem, a technique in which a contact pressure of the cleaning blade is increased to remove toner particles is proposed. However, this technique accelerates abrasion of the photoreceptor and promotes peeling of the protective layer. Because of these reasons, a need exists for a photoreceptor having a durable protective layer which hardly peels off, which can be used for electrophotographic image forming processes using a polymerization toner.